Generally, nuclear fuel assemblies comprise nuclear fuel rods and a support skeleton having two nozzles, guide tubes interconnecting the nozzles, and spacer grids for holding the rods.
Each spacer grid comprises two sets of crossed plates and an outer belt, thus defining cells, some of which have guide tubes passing through them and others have fuel rods passing through them. The plates are provided with means for holding the rods at the nodes of a substantially regular array and they are secured to at least some of the guide tubes.
At least one of the spacer grids also serves to support the rods. For this purpose, it is usually provided with springs that are cut out in the plates or that are fitted to the plates, and that serve to press the rods against dimples stamped in the plates and forming the opposite faces of the cells.
The other grids serve only to hold the rods at the nodes of the array. To do this, they present dimples on each of the faces of a cell having a rod passing therethrough, the dimples serving to press against the rod.
French patent No. 2 665 291 also discloses additional mixer grids for interposing between the spacer grids and having fins for improving the mixing of the cooling fluid flowing through the assemblies.
Once manufactured, such assemblies extend rectilinearly and vertically along a direction that is referred to as being “axial”. Once in place in a reactor, these assemblies deform because of the irradiation and can take on C-shapes, S-shapes, or W-shapes.
Such deformations lead to numerous problems. In operation, they make it more difficult to insert control and shutdown clusters into the guide tubes.
During handling, these deformations increase the risk of assemblies catching on one another, e.g. during operations of loading or unloading the core of the reactor.